Boost for reddit allowed the creation of custom feeds with groups of subreddits, I suspect this is harder across lemmy instances but it would make my many communities easier to navigate if I could group them together (even better if they can be grouped independent of whether I have actually joined the community).
Yeha, this is definetely not the responsibility of Boost. Custom Feeds are data that need to be stored in a database. If this is done in the local storage, then they’d dissappear if you switch devices. So, Boost would need to create a data layer somewhere in the cloud, and that’s certainly not going to happen.